Dual SIM Dual Standby (DSDS) phones (i.e., DSDS User Equipments (UEs)) are designed to share a single radio frequency (RF) between two stacks (i.e., stack-1 and stack-2). A DSDS UE may communicate using radio link control (RLC) during an uplink (UL) and downlink (DL). When the stack-1 of the DSDS UE is performing a high-speed downlink packet access (HSDPA) or Long Term Evolution (LTE) data transfer, the RF is continuously required by the stack-1. However to maintain paging reception, measurements and signalling on the stack-2 of the DSDS UE, RF usage gaps (i.e., DSDS gaps) are temporarily created for the stack-1 (as shown in FIG. 1, block (b)) in either a scheduled or unscheduled fashion.
Physical Layer Synchronization:
In a connected mode, the DSDS UE continuously decodes a dedicated physical control channel (DPCCH) to maintain synchronization with the network, and vice versa, If the DPCCH signal to interference ratio (SIR) drops below a set threshold (Qout) (as shown in FIG. 1, block (a)) the stack-1 physical layer starts reporting Out of Sync (OOS) to a resource controller and ceases the transmission path i.e., no uplink dedicated physical control channel UL-DPCCH transmission. If consecutive OOS indications are received N313+T313 times, the resource controller releases the resource and starts a recovery mechanism. In case of DSDS due to non-contiguous RF availability, the DPCCH decoding, at the DSDS UE, for all Radio frames is not possible and the network will receive noise in the UL-DPPCH for RF void intervals.
Because the network is unaware of the DSDS RF gaps, the network considers the noise as the actual information and therefore attempts to decode the noise as a UL-DPCCH. This results in a drop in UL-DPCCH average and a SIR drop. If RF gaps are of considerable duration, this may result in switching off network transmission falsely (as shown in FIG. 1, block (d)). In such cases, stack-1, after an RF resumption (as shown in FIG. 1, block (c)), observes noise in the DL-DPCCH and in turn ceases its transmission after a few frames (as shown in FIG. 1, block (e)). Therefore, it can be easily observed that RF communication between the DSDS UE and the network is halted. In such cases DSDS UE can recover only after expiration of a timer such as an N313+T313 timer.
Link Layer Downlink Protocol Data Unit (PDU) Miss:
Consider a scenario in which the network is trying to send the data packet i.e., one service data unit (SDU), to the DSDS UE. The SDU is segmented into multiple Protocol Data Units (PDUs) (e.g., three PDUs as shown in FIG. 2 (a)). The essential parameter length indicator (LI) is added to the last PDU, as the LI indicates the length of the PDUs (LI=3) which in turn indicates completion of the SDU transmission from the network.
As discussed above, in order to maintain paging reception, measurements and signalling on the stack-2 of the DSDS UE, RF usage gaps (i.e., DSDS gaps) are created temporarily for the stack-1.
Referring to the FIG. 2, consider if the network has transmitted (S202, S204 and S206) RLC PDU SN1, RLC PDU SN2 and the RLC PDU SN3 to the DSDS UE. As seen in the FIG. 2, the DSDS UE has only received the RLC PDU SN1, and fails to receive the RLC PDU SN2 and the RLC PDU SN3 due to the RF gap.
In such a case, detection of missed PDUs (i.e., the RLC PDU SN2 and the RLC PDU SN3) will happen when poll timer at the network expires, in which case the network retransmits the missed PDUs through an acknowledgement (ACK)/negative acknowledgement (NACK) procedure, described below:    Once the poll timer expires, the last PDU i.e., RLC PDU SN3 is retransmitted (S208) to the DSDS UE.    Further, the DSDS UE transmits (S210) the NACK for RLC PDU SN2 and ACK for RLC PDU SN3, based on the ACK/NACK status received the network can retransmit (S212) the RLC PDU SN2 to the DSDS UE. Further, the DSDS UE transmits (S214) the ACK for the RLC PDU SN2 to the network.
As seen above, the retransmission of the missed PDUs is entirely dependent on the poll timer at the network side. However, the DSDS UE can possibly move into next RF pause by the time the poll timer expires at network side, also the poll timer value can be as high as 2000 ms.
Signalling Response Round Trip Time Delay:
Consider a scenario in which the DSDS UE trying to transmit the data packet i.e., one service data unit (SDU), which is segmented into multiple PDUs (e.g., three PDUs as shown in FIG. 3), to the network. Because the signalling SDU is segmented into multiple PDUs, the DSDS UE can take multiple transmission time intervals (TTIs) to complete transmission of the SDU. The essential parameter length indicator (LI) is added to the last PDU, as the LI indicates the length of the PDUs (for example LI=3) which in turn indicates completion of the SDU transmission.
If the RF pause happens during the transmission of the SDU and the SDU transmission is not yet completed, pending segments of the SDU will be transmitted after the RF resumes. For example, referring to the FIG. 3, the DSDS UE transmits (S302) the RLC PDU SN1 and incurs the RF pause (RF outage) thereby the transmission (S304 and S306) of the RLC PDU SN2 and the RLC PDU SN3 are interrupted. The DSDS UE then retransmits (S308 and S310) the RLC PDU SN2 and the RLC PDU SN3 when the RF resumes. This will increase the signalling response round trip time.
In another example, if the RF pause happens after transmission of initial segments of a Measurement Report with Event 1A/1B, then until RF resumes, the measurement report will not reach the network. This will delay the start of event 1A/1B related handling procedure at the network side. As a result, addition/deletion of the reported cell to active set will be delayed.